1. Field of the Invention
The present invention relates to wireless local area networks (WLANs) and in particular to various techniques that ignore or cancel spurs, thereby improving receiver performance.
2. Discussion of the Related Art
Wireless local area networks (WLANs) are becoming increasingly popular as communication networks. The IEEE 802.11 standards provide guidelines for the operation of devices operating in WLANs. To address multipath and other conditions, a wireless system can employ various techniques. One such technique is Orthogonal Frequency Division Multiplexing (OFDM).
In an OFDM system, a signal can be split into multiple narrowband channels (called sub-channels) at different frequencies. For example, current 802.11a and 802.11g OFDM systems include 52 sub-channels. Thus, a transmitted signal could be represented by x−26 . . . x−1, x1 . . . x26, wherein both negative and positive side frequencies are included. In this configuration, each sub-channel carries a portion of the signal. Each sub-channel is “orthogonal” (i.e. independent) from every other sub-channel. Multipath conditions and noise can result in deterioration of this orthogonality.
In an attempt to restore orthogonality, the 1999 IEEE 802.11a standard provides that a transmitted data packet includes a preamble, which precedes the actual data. Note that all references to the 802.11a standard, which operates in the 5 GHz band, equally apply to the newer 802.11g standard, which operates in the 2.4 GHz band. FIG. 1 illustrates a portion of a data packet 100 including a preamble 105. As defined in the 802.11a standard, preamble 105 includes 10 “short” identical known symbols 101A-101J of 0.8 μsec (hereinafter shorts 101) concatenated to 2 “long” identical known symbols 102A-102B of 3.2 μsec (hereinafter longs 102). Note that a symbol refers to any waveform at discrete moments in time (e.g. represented as voltage versus time).
Longs 102 can be used to provide channel estimation. Specifically, because longs 102 are known, the receiver can use these symbols to provide channel estimations for a subsequent data symbol 103 in the data packet. In this manner, longs 102 can thereby increase the likelihood that the received data symbols can be correctly interpreted. Longs 102 are also called “training” symbols because they can “train” an equalizer, e.g. a frequency domain equalizer, to learn about channel conditions.
The 802.11a standard also provides that guard intervals (GIs) should be placed before longs and data. Specifically, a double guard interval (GI2) is placed before long symbols 102A-102B, thereby forming part of longs 102. In contrast, a regular guard interval (GI) is placed before data 103A, thereby forming part of data symbol 103. The double guard interval, as the name implies, is twice as long as the regular guard interval.
Shorts 101 can be used to determine a frequency offset between the oscillators in the receiver and transmitter. Additionally, shorts 101 can be used to provide initial system time synchronization. System time synchronization can also be continuously tuned using the data symbols.
A difference between the frequency of the transmitter and receiver oscillators can adversely and significantly impact system performance. For example, if the receiver's clock is not aligned with the incoming data, then sampling of the received signal could be sub-optimal. Additionally, phase noise in the radio-frequency synthesizers in the transmitter or receiver can degrade performance. For this reason, “pilots”, also known signals (e.g. −1 and 1 in a pre-determined pattern) defined by the 802.11a standard, are provided on 4 of the 52 orthogonal sub-channels to track and correct the difference between clocks.
For example, FIG. 2 illustrates a data symbol 200 including a GI 201 and data 202. If the receiver's clock samples earlier in time than the incoming data, then instead of detecting the values indicated by 1st sampling 203, the values indicated by 2nd sampling 204 could be detected. This de-synchronization can result in a phase ramp 300 in the frequency domain, as shown in FIG. 3. Note that phase ramp 300 is negative when the receiver's clock samples earlier in time than the incoming data and positive when the receiver's clock samples later in time than the incoming data. Because of the continual “slide” in sampling (see FIG. 2), the slope of the phase can continue to rotate symbol by symbol.
Moreover, when a signal is transmitted, the signal is modulated by the channel frequency, thereby improving its propagation properties in the channel. The modulation is based on the clock at the transmitter. Thus, at the receiver, the signal must be demodulated. This demodulation can result in some residual phase error, which can be represented by an offset 301. The 4 pilots provided by the 802.11 standard are used to track the phase slope and phase offset, thereby allowing the system to compensate for such slope and offset when necessary.
Unfortunately, using only four sub-channels can be insufficient to compensate for phase slope and offset. For example, certain narrow tones, called “spurs” can corrupt the pilots provided on these four sub-channels, thereby distorting the phase slope and phase offset information derived from such pilots. The spurs can be generated by an oscillator and/or synthesizer provided in the receiver, as now described in reference to FIG. 4A.
WLAN Receiver: Overview
FIG. 4A illustrates a simplified receiver 400 for receiving signals in a WLAN environment. In receiver 400, a bandpass (BP) filter 402 receives the incoming signals from an antenna 401 and outputs a predetermined band of frequencies (while excluding those frequencies higher and lower than the predetermined band). A variable gain RF amplifier 403 can provide an initial amplification to that predetermined band of frequencies. A mixer 404 converts those amplified signals into intermediate frequency (IF) signals, which are then amplified by an IF amplifier 405.
At this point, mixers 406 and low pass filters 407 (including both I and Q branches) can generate signals in the desired channel (called the baseband signals). Amplifiers 408 then amplify these baseband signals. Analog to digital converters (ADCs) 410 (provided for both the I and Q branches of low pass filters 407) transform the amplified baseband signals into digital signals that can be analyzed by a processing block 411. Processing block 411 determines the modulation type of the detected signal and provides this determination to an appropriate decoder in a decoder block 412 for system optimization, thereby allowing the originally transmitted signal to be recovered.
Of importance, a reference oscillator 420 in receiver 400 provides its clock to synthesizers 423 and 424 as well as to other components in receiver 400. Synthesizers 423 and 424 generate local oscillations 421 and 422, respectively, using the frequency of reference oscillator 420. For example, synthesizer 423 could generate a 2.4 GHz frequency for local oscillation 421 from a 32 MHz frequency provided by reference oscillator 420. Note that in some embodiments, synthesizers 423 and 424 can be combined into a single synthesizer capable of generating multiple frequencies.
Unfortunately, the frequency of reference oscillator 420 (a square wave signal) creates harmonics (both odd and even) of the reference frequency (e.g. 32 MHz×n). These harmonics can include spurs, i.e. known frequency spectra unrelated to a received signal, which can adversely affect the front end of receiver 400, particularly RF amplifier 403. Spurs can be present at harmonics near the desired received signal.
For example, if the radio is to tune to 2.4 GHz, then synthesizer 423 could be set to 1.92 GHz and synthesizer could be set to 0.48 GHz (because 1.92 GHz=0/48 GH=2.4 GHz). If the synthesizers generate these frequencies from a 32 MHz reference oscillator, then spurs can be generated at 2.4 GHz (32 MHz×75), 2.432 GHz (32 MHz×76), and 2.464 GHz (32 MHz×77). FIG. 4B illustrates an exemplary spur 430 generated at 2.432 GHz. Of importance, spur 430 coincides with a pilot 431 (one of four pilots indicated by a cross-sectional pattern) provided within the 52 sub-channels of this 17 MHz wide band. Note that although spur 430 is a narrow band frequency, the strength of spur 430 can affect other sub-channels adjacent to the sub-channel including spur 430 as indicated by curves 432 (also known as skirts). Other spurs, not shown, could coincide with and/or affect other pilots, data, and the shorts/longs in the preamble.
Of importance, a spur is mixed with a signal received by antenna 401. Therefore, spurs cause significant problems with signal detecting, amplifier gain adjustment, and signal decoding. Thus, a need arises for a technique of mitigating the effects of such spurs, thereby improving the performance of the receiver.